For a case where data to be retained, referred to below as object data, is retained in a plurality of sites for placement, there is known such a technique that the object data is retained in a substantially evenly distributed state in the individual sites for placement. By using such technique, and simultaneously accessing a plurality of sites for placement, it is possible to improve the performance of access to the object data.
However, in accessing the object data, retained distributed in the multiple sites for placement, by such technique, it is necessary to search the placement site retaining the object data, that is, to identify the site for placement retaining the object data being accessed.
In Patent Literature 1 and in Non-Patent Literatures 1 to 4, there are disclosed methods in which the relationship of correspondence between the object data and the sites for placement is determined on the basis of the information proper to the object data using a preset algorithm. Such information is referred to below as an identifier. In the method employing the identifier, the above mentioned algorithm is used in accessing the object data to decide from such identifier on the placement site in which the object data is stored.
There exist two sorts of methods to check for the placement site holding the object data on memory with the use of the above mentioned algorithm.
In the first method, a device executing the method of deciding on the placement site holding on memory the object data, referred to below as a decision device, stores the total of the sites for placement likely to be holding the object data on memory.
In the second method, the decision device stores a part of the sites for placement likely to be holding the object data on memory. This second method is disclosed in the Non-Patent Literatures 1 and 2.
[Patent Literature 1]
    JP Patent Kokai Publication No. JP2007-073003A